Short Answer

A child must be at least 24 months old to qualify for a semi-private class. The minimum age is 2 years old for safety. This is very rarely a grey area.

The More You Know

Talk about safety first, decision making at that age/level of development, more emotionally prepared for that transition (even if it's tough), even body size and being able to reach the step/island, and if they were really reaching I might talk about understanding of language/comprehending and applying verbal directions.